Efficient use of space segment is a key factor in the business of communication via satellite. Over the last several years, much attention was given to improving the economics of interactive data networks over satellite.
In one example of an interactive data network over satellite, a central hub may communicate with multiplicity of remote terminals, for example, by transmitting a statistically multiplexed forward signal. For several reasons, the remote terminals often receive this forward signal at different signal to noise ratios (SNR). In many cases, the remote terminals may be dispersed over a large geographical area with uneven satellite illumination intensity or coverage. In some cases, some terminals may suffer from antenna pointing error and/or from rain attenuation, both may have negative impact on reception quality.
Several years ago, the Digital Video Broadcasting for Satellite (DVB-S) standard was presented. However, a forward signal (e.g. in an interactive data network over satellite) implemented using DVB-S may require additional power in order to have sufficient link margin for maintaining high link availability. Some years later, the DVB-S2 standard was introduced and with it the concept of adaptive coding and modulation (ACM). In the adaptive mode, data destined to a specific remote terminal may be modulated and coded (i.e. for error correction) in accordance with the momentary SNR of the forward signal as received at the destination remote terminal. According to the DVB-S2 standard, the transmitted signal is made up of a stream of blocks (Baseband Frames (BBFRAME)), where each block may be transmitted using a different combination of modulation and error-correction code-rate (often denoted as MODCOD). The remote terminals periodically communicate (e.g. over a return link) their forward link reception SNR to the central hub, allowing the data destined to them to be transmitted over the forward link using an optimal MODCOD. Consequently, there is no need to keep a global link margin according to worst case rain and/or reception conditions. At any given time the percentage of terminals that may experience a bad link is usually quite small hence the overall data throughput from the central hub to the remote terminals is maximized.
Small aperture antennas often have a low G/T (Gain over Temperature) property. Consequently, a remote terminal operating with a small aperture antenna may receive a forward link signal transmitted from a hub at an SNR too low for using DVB-S2 ACM. FCC (Federal Communication Commission) and ETSI (European Telecommunication Standard Institute) regulations limit EIRP (Effective Isotropically Radiated Power) per predefined frequency bandwidth at the satellite. In the United States for example, it is often not possible to receive a DVB-S2 ACM signal using a dish antenna of a diameter below approximately 30 centimeters without exceeding said EIRP density limitations. Moreover, small antennas have relatively wide beams, which may result in receiving interferences from neighboring satellites (i.e. satellites located in relative proximity to the satellite of interest). On the other hand, small antennas are popular in some applications, such as communication on the move, i.e. for mobile terminals. For a mobile terminal, using a small dish antenna may be a critical requirement, for example, due to aerodynamic considerations, weight considerations, availability of installation surface (like on a roof of a vehicle), cost, etc.
In order to allow reception of a signal at low and even at negative SNR levels, some satellite communication systems make use of spread-spectrum techniques. Spread-spectrum techniques allow transmission of a signal with sufficient energy per transmitted data bit (hence allowing reception of the signal with very low gain antennas and/or at very bad link conditions) while keeping the power spectral density (i.e. EIRP per bandwidth) at the satellite within the limits set by the applicable regulations. Using constant spectrum spreading techniques in combination with robust modulations, e.g. BPSK (Binary Phase-Shift Keying), may allow operation at very low SNR levels (e.g. −15 dB and in some cases even lower than that when using spreading factors above 15 dB).
However, where spreading is used (including in some embodiments of DVB-S2, such as that which is defined in EN 301790, version 1.5.1, section 5.1), the spreading factor is fixed, i.e. the same spreading factor is used for all the information being transmitted. Consequently, in point-to-multipoint applications, the forward signal may be inefficient, as it may be necessary to set the fixed spreading factor high enough in order to have enough spreading gain for demodulation at worst case reception conditions of any of the remote terminals, although typically only a small portion of these remote terminals may experience a bad link (due to rain, for instance). Similar inefficiency may also be experienced in point-to-point applications where operation at low SNR is needed.
It is important to note that optimal efficiency is typically achieved when the spreading factor is set to the minimal value yielding a spreading gain sufficient for demodulation at the lowest modulation constellation and the lowest coding rate supported by a terminal. Hence, in order to maximize forward channel throughput, data towards terminals with bad reception conditions may optimally be sent with a higher spreading factor than data towards terminals with better reception conditions.